1. Field of the Invention
The present invention relates to LCD (Liquid Crystal Display) panel substrate fabrication, and more particularly to an exposure method and system for fabricating an LCD panel substrate without stitch defects.
2. Description of the Prior Art
The exposure apparatus used for fabricating a liquid crystal display device substrate is generally either a step-and-repeat apparatus or a step-and-scanning apparatus. The step-and-repeat apparatus sequentially exposes an exposure region of a unit cell, while the step-and-scanning apparatus transcribes a reticle or a mask pattern onto each exposure region of an array substrate by synchronously moving the reticle or the mask and a substrate in the same direction.
FIG. 1 shows a conventional projective exposure steppe-and-repeat apparatus for fabricating an LCD substrate. As shown in FIG. 1, an LCD pattern on a reticle or a mask (hereafter, “reticle”) 103 is illuminated by an illuminating optical system 102, and is exposed onto a predetermined exposure area on a plate 106. The plate 106 is typically a rectangular glass substrate placed on an XY stage 105.
When a pattern is transferred by the exposure, the plate 106 is moved by a predetermined distance by moving the XY stage 105. The LCD pattern is then exposed onto a new exposure area. Thus, the process of exposing the LCD pattern may occur multiple times in multiple different locations on the plate 106.
When a new pattern from a new reticle is needed, the reticle 103 is exchanged for another reticle by the reticle exchanging mechanism 110. Again, the LCD pattern on the new reticle is sequentially exposed onto a predetermined exposure region a predetermined number of times. As a result, multiple reticle patterns are transferred to the plate 106 at multiple locations.
In the above-described step-and-repeat exposure apparatus, the position of the plate 106 on the XY stage 105 is correctly monitored by a laser interferometer 107, and the coordinates of the position are determined. The alignment of the reticle 103 is performed by a reticle alignment system 108, and the alignment of the plate 106 is performed by a plate alignment system 109.
FIG. 2 shows an example of four LCD substrate patterns transferred to the plate 106 by the exposure apparatus of FIG. 1. As shown in FIG. 2, in transferring the entire LCD substrate pattern, the overall pattern is divided into, for example, six pattern regions labeled A, B, C, D, E and F. At each boundary portion between patterns (as examples, boundary 202 or boundary 204), a small amount of overlap exposure is carried out as the patterns are exposed onto 6 different positions. In the example shown in FIG. 2, each pattern region A, B, C, D, E, and F is exposed through one of six different reticles that holds the appropriate pattern for the pattern region. The six patterns are generated by repeatedly exposing and replacing reticles, thereby forming an entire LCD substrate pattern, four of which are shown in FIG. 2.
In exposing a reticle according to the above-described method, there is often an alignment discrepancy that occurs at boundary portions between patterns. FIG. 3 shows two examples of alignment discrepancy that may result from alignment error of the reticle and the plate, the distortion of the projective optical system, and other imperfections in the system.
A examples, when a pattern to be transferred is subject to a rotation error, or when there is a positional error with respect to the positions of the patterns A and B to be transferred, then there is an alignment discrepancy at the boundary of the exposed patterns as shown in FIG. 3. An additional source of alignment discrepancy or overlap error is projection lens distortion in the system.
When any alignment error occurs, there arises the problem that the LCD substrate generated by the exposure process does not have the characteristics that it was designed to have. The LCD substrate is therefore unusable for an LCD display, and represents a waste of time and materials. Not only the step-and-repeat apparatus, but also the step-and-scanning apparatus experience the same problem.
Recently, LCD panels have significantly increased in size, but the size of the exposure region covered by any particular mask has not. Thus, more exposure cycles are necessary to expose the LCD substrate. Accordingly the problem has worsened, even as commercial demand for LCD displays is increasing.
In other words, because an LCD display incorporates a large LCD substrate, exposure has to be performed many times to fabricate the LCD substrate. In an attempt to prevent alignment discrepancy, there have been attempts to carry out exposure using a minute overlap portion between exposures.
Nevertheless, in those attempts, pattern alignment discrepancy occurs between an overlapped region and a non-overlapped region. In other words, in some instances, exposure happens twice in the minute overlap region, exposure happens only once in another region, and pattern discrepancy occurs between the two regions after etching. After completing the fabrication of the liquid crystal display device, the pattern discrepancy looks like a stitch, and accordingly the screen is defective.
Thus, there is a need to address the problems noted above, and others previously experienced.